Recently, together with the wide spreading of the internet and computers and the rapid distributions of multimedia data, illegal copies (piracy) and distributions are widely prevalent so that an effective protection apparatus for a copyright to multimedia data gets required. Watermarking technology is one that embeds user information (watermark), unrecognizable by a user, in multimedia data, to thereby prevent pirated copies and protect a copyright of a copyright owner.
The watermark means a mark developed in a step using a frame for pressing wet fibrous material to get rid of water in a process making paper from papyrus in ancient times. Marks embedded in paper in order for paper manufacturers in the middle ages to prove their own goods are the watermarks in the middle ages, and, nowadays, an image is embedded which can be recognized only with light when, in a process of making banknotes, printing on both sides of a sheet of paper after drying the wet sheet on which printing has been done, and the image is referred to as a watermark.
In these days, together with the increase of digital media, the concept of a digital watermark has appeared. Just as paper in an analog concept is substituted with the concept of digital paper, digitalizing all the analog media in which the past watermarks were embedded has brought into the concept of the digital watermark as a mark hidden in digital images, audio, video, and so on. That is, the watermarking refers to all technical methods hiding and extracting a special form of watermark in multimedia contents in order to protect a series of multimedia contents. At the beginning, researches have been carried out for methods hiding original multimedia contents themselves, but, at present, it is a trend that strong watermarking technologies using lots of technical transform methods are developing.
The watermarking is classified into a visible watermarking and an invisible watermarking based on the visibility of a watermark, and the invisible watermarking is again classified into a spatial domain watermarking and a frequency domain watermarking based on the methods embedding a watermark.
The visible watermarking specifies a copyright by embedding in an original image author information which can be recognized with eyes. The visible watermarking can be used with ease but has a drawback in that the originals are damaged.
Accordingly, the invisible watermarking is primarily used in the image watermarking technology in these days. The invisible watermarking is a technology embedding a watermark not to be visually perceived by using a limit of senses of the human visual system. While the spatial domain watermarking embeds and extracts a watermark with ease, there is a high possibility to lose a watermark by means of signal processing, video processing (non-linear filtering, rotating, cutting, moving, enlarging, and reducing transforms and the like), and compressing.
However, the frequency domain watermarking employs transform techniques such as Fourier transform, discrete cosine transform, or the like for embedding and extraction, so there exists a drawback in that it has a complicated algorithm and requires lots of arithmetic operations, but it has an advantage in that it is robust on general attacks such as filtering or compressions.
The invisible embedding of a watermark requires an embedding of the same in a low value on a broad area, which is carried out by the spread spectrum technology of Ingemar J. Cox. In the spread spectrum technology, a pseudo-random sequence is used as a watermark, which is a method that can be effectively used since the sequence has a uniform distribution function and is evenly distributed over the entire bandwidth of frequencies.
For methods transforming an original image into a frequency domain, the fast Fourier transform (FFT), discrete cosine transform (DCT), and wavelet transform are generally utilized a lot, which takes a method embedding and restoring a watermark into the original state in a transform plane. However, the method has a high possibility to lose a watermark on attacks such as image rotating, cutting, moving, enlarging, reducing, or the like.
As stated above, the watermarking methods in the spatial domain or frequency domain have advantages and disadvantages in their own ways. For an alternative, a watermarking method using the log-polar mapping and Fourier transform has been developed to compensate for the loss of a watermark, which is the weak point of the frequency domain watermarking method, in rotating, enlarging, or reducing an image. The method converts rotations, enlargements, and reductions into a simple movement forms through the log-polar mapping and detects a watermark by using the characteristics that the amplitudes of the Fourier transform are invariable with movements. However, the method is weak at the video processing such as compressions as well as has a big drawback in that the loss due to the log-polar mapping itself is very high and the implementations become very complicated.
As mentioned above, the developed watermarking technologies for video have advantages and disadvantages in general in their own ways. Further, the pseudo-random sequence watermark being widely used at present can confirm what key value a watermark embedded in an image has, but has difficulties in embedding and extracting various copyright information.
Further, in case of firstly casting and then embedding a watermark in an input image, an embedded watermark is changed if the image undergoes rotations, partial cuttings, or the like, causing a problem impairing copyright information.